Skeletal Muscle. Electron microscopy and cytochemistry will be used to analyze denervated and differentiating muscle fibers of the rat diaphragm. The relationship of ultrastructural manifestations of protein synthesis to increased acetylcholine sensitivity will be explored further. The exact sequence of ultrastructural changes that occur following denervation of the adult diaphragm will be analyzed, and investigations of the ultrastructural response to pharmacological agents that affect acetylcholine sensitivity will be continued. The postnatal development of the neuromuscular association and its relationship to the differentiation of three fiber types is being studied at the ultrastructural level. Uterine and placental Differentiation. Cyclic endometrial differentiation is being analyzed in albino rats with a 4 day estrous cycle by cytochemistry, electron microscopy, and radioautography. Special emphasis is placed on the ultrastructural differentiation that occurs in the luminal epithelium preceding estrus. Uterine and placental differentiation in the bandicoots (Perameles nasuta and Isoodon macrourus) will be analyzed by light and electron microscopy, and comparisons will be made with that occurring in the opossum (Didelphis marsupialis virginiana). Particular attention is given to definition of the nature of the chorio-allantoic placental barrier in the bandicoot. A cytochemical and ultrastructural analysis of ovarian- uterine relationships during anestrus in the opossum has been initiated.